The proposed work will develop the hypothesis that the concentration of acetylcholine (ACh) at post junctional effector sites in the atrium regulates the expression of beta-adrenergic cyclic AMP-mediated functions. Isoproterenol-stimulated cyclic AMP accumulation in isolated murine atria is markedly inhibited by K ion-induced depolarization, cholinesterase inhibition or carbachol. All of these manipulations block cyclic AMP accumulation through a muscarinic (atropine-sensitive) mechanism. I propose to demonstrate that elevated K ion releases ACh from pre-labelled (with 3H-choline) neurotransmitter stores in isolated atria, and that subsequent inhibition of cyclic AMP accumulation is proportional to the available released ACh. By quantitation of the effect of released or exogenous ACh, I will determine whether there is noncompetitive antagonism of catecholamine-stimulated cyclic AMP accumulation by muscarinic receptor activation. Isoproterenol-induced activation of protein kinase and phosphorylase will also be shown to be inhibited in proportion to the concentration of choline ester. I will examine local presynaptic mechanisms by which drugs and hormones might modulate K ion-induced ACh release, as potential physiological regulators of the expression of cyclic AMP-dependent beta-adrenergic functions. Developmental and hormonal changes in the density or sensitivity of muscarinic receptors will be utilized to examine post-synaptic cholinergic receptor mechanisms regulating adrenergic responses. Other studies will focus on changes in cyclic GMP, phosphodiesterase, and GTPase activity as possible bases for muscarinic antagonism of cyclic AMP accumulation. The possibility that elevated K ion releases ACh, and thereby modulates beta-adrenergic responses, in papillary muscle and in perfused hearts will also be examined. The eventual goals of this work will be to demonstrate that the dominance of parasympathetic influences in the atrium can be explained by cholinergic inhibition of cyclic AMP accumulation, and that intracardiac cholinergic neurons are critical effector sites for drug and hormone actions on cardiac muscle.